1. Field of the Invention:
The present invention relates to a toner for developing electrostatic latent images formed in electrophotographic process, electrostatic printing process, electrostatic recording process and the like.
2. Description of the Prior Art:
Electrostatic latent images are developed by toners comprising a binder and a coloring agent to form a toner image, which is fixed either immediately or after being transferred onto a printing sheet. Among the several methods currently used to fix the toner image, contact type heat fixing methods such as heat roller type fixing is preferred to non-contact type heat fixing methods such as heat plate type fixing because of high thermal efficiency and the capability of high-speed fixing. In contact type heat fixing, the toner must be made soft enough by the heated rollers to be fixed permanently, and "an offsetting phenomenona" wherein part of the toner adheres to the surface of the heated rollers must be prevented.
Japanese Patent Application (OPI) No. 134652/75 (the symbol OPI as used herein means an unexamined published Japanese patent application) proposes to meet these requirements by using a toner the binder component of which is a resin made of a mixture of low-molecular and high-molecular weight polymers. Because of the presence of the high-molecular weight polymer, this toner is effective in preventing offsetting, but it has a relatively high softening point and requires considerably high temperatures to effect the desired fixing. This causes various problems in practice, such as increased energy consumption, prolonged warming up of the rollers, and the need for using rollers having high heat resistance and durability. If the proportion of the low-molecular weight polymer in the binder is increased in an attempt to reduce the softening point of the toner, its ability to prevent offsetting is reduced, and in addition, its glass transition point is also decreased, thus increasing the chance of aggregation of the toner particles during storage or within the developer housing.
Japanese Patent Publication No. 23354/76 proposes preventing a offsetting by using a crosslinked resin as a toner binder. But if the crosslinking density of the resin is increased to the level necessary for preventing the offsetting, the fixing temperature is increased and the same problems arise as in the case of the toner of Japanese Patent Application (OPI) No. 134652/75. As a further disadvantage, a resin with a three-dimensional network is ground only with difficulty into particles for incorporation in the toner.
Both the high-molecular weight polymer of Japanese Patent Application (OPI) No. 134652/75 and the crosslinked resin of Japanese Patent Publication No. 23354/76 are very hard, so with a toner using them as the binder component, the heated rollers deteriorate with fatigue to such an extent that offsetting easily occurs. To prevent this problem, a mold releasing lubricant such as silicone oil may be supplied to the surface of the rollers, but the oil emits a malodor when heated, or fouls the interior of the fixing apparatus. Furthermore, the oil supplying device complicates the overall construction of the fixing apparatus.